add C API for epoch_deadline_callback and wasmtime_error_t creation (#6359)

* add C API for epoch_dead_callback and wasmtime_error_t creation

* revise C API wasmtime_error_new msg encoding

crates/c-api/include/wasmtime/error.h

@@ -30,6 +30,11 @@ extern "C" {
  */
 typedef struct wasmtime_error wasmtime_error_t;
 
+/**
+ * \brief Creates a new error with the provided message.
+ */
+WASM_API_EXTERN wasmtime_error_t *wasmtime_error_new(const char*);
+
 /**
  * \brief Deletes an error.
  */

crates/c-api/include/wasmtime/store.h

@@ -206,15 +206,30 @@ WASM_API_EXTERN wasmtime_error_t *wasmtime_context_set_wasi(wasmtime_context_t *
 
 /**
  * \brief Configures the relative deadline at which point WebAssembly code will
- * trap.
+ * trap or invoke the callback function.
  *
  * This function configures the store-local epoch deadline after which point
- * WebAssembly code will trap.
+ * WebAssembly code will trap or invoke the callback function.
  *
- * See also #wasmtime_config_epoch_interruption_set.
+ * See also #wasmtime_config_epoch_interruption_set and
+ * #wasmtime_store_epoch_deadline_callback.
  */
 WASM_API_EXTERN void wasmtime_context_set_epoch_deadline(wasmtime_context_t *context, uint64_t ticks_beyond_current);
 
+/**
+ * \brief Configures epoch deadline callback to C function.
+ *
+ * This function configures a store-local callback function that will be
+ * called when the running WebAssembly function has exceeded its epoch
+ * deadline. That function can return a #wasmtime_error_t to terminate
+ * the function, or set the delta argument and return NULL to update the
+ * epoch deadline and resume function execution.
+ *
+ * See also #wasmtime_config_epoch_interruption_set and
+ * #wasmtime_context_set_epoch_deadline.
+ */
+WASM_API_EXTERN void wasmtime_store_epoch_deadline_callback(wasmtime_store_t *store, wasmtime_error_t* (*func)(wasmtime_context_t*, void*, uint64_t*), void *data);
+
 #ifdef __cplusplus
 }  // extern "C"
 #endif

crates/c-api/src/error.rs

@@ -14,6 +14,20 @@ impl From<Error> for wasmtime_error_t {
     }
 }
 
+impl Into<Error> for wasmtime_error_t {
+    fn into(self) -> Error {
+        self.error
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn wasmtime_error_new(
+    msg: *const std::ffi::c_char,
+) -> Option<Box<wasmtime_error_t>> {
+    let msg_string = String::from_utf8_lossy(unsafe { std::ffi::CStr::from_ptr(msg).to_bytes() });
+    Some(Box::new(wasmtime_error_t::from(anyhow!(msg_string))))
+}
+
 pub(crate) fn handle_result<T>(
     result: Result<T>,
     ok: impl FnOnce(T),

crates/c-api/src/store.rs

@@ -106,6 +106,48 @@ pub extern "C" fn wasmtime_store_new(
     })
 }
 
+// Internal structure to add Send/Sync to the c_void member.
+#[derive(Debug)]
+pub struct CallbackDataPtr {
+    pub ptr: *mut c_void,
+}
+
+impl CallbackDataPtr {
+    fn as_mut_ptr(&self) -> *mut c_void {
+        self.ptr
+    }
+}
+
+unsafe impl Send for CallbackDataPtr {}
+unsafe impl Sync for CallbackDataPtr {}
+
+#[no_mangle]
+pub extern "C" fn wasmtime_store_epoch_deadline_callback(
+    store: &mut wasmtime_store_t,
+    func: extern "C" fn(
+        CStoreContextMut<'_>,
+        *mut c_void,
+        *mut u64,
+    ) -> Option<Box<wasmtime_error_t>>,
+    data: *mut c_void,
+) {
+    let sendable = CallbackDataPtr { ptr: data };
+    store.store.epoch_deadline_callback(move |mut store_ctx| {
+        let mut delta: u64 = 0;
+        let result = (func)(
+            store_ctx.as_context_mut(),
+            sendable.as_mut_ptr(),
+            &mut delta as *mut u64,
+        );
+        match result {
+            Some(err) => Err(wasmtime::Error::from(<wasmtime_error_t as Into<
+                anyhow::Error,
+            >>::into(*err))),
+            None => Ok(delta),
+        }
+    });
+}
+
 #[no_mangle]
 pub extern "C" fn wasmtime_store_context(store: &mut wasmtime_store_t) -> CStoreContextMut<'_> {
     store.store.as_context_mut()